Extraction of alumina from ores

ABSTRACT

Alumina in increased yeild is recovered from aluminiferous ores containing titanium and also iron in goethite phase by subjecting the ores to a wet caustic digestion step at temperatures in excess of about 280*C in admixture with a calcium compound added in an amount at least sufficient to convert substantially all of the titanium to a calcium-titanium compound for a time sufficient to accomplish the conversion of goethite to hematite followed by conventional alumina recovery steps.

United States Patent [191 King [ 1 June5,1973

[54] EXTRACTION OF ALUMINA FROM ORES [75] Inventor: William R.King,Cupertino, Calif.

[73] Assignee: Kaiser Aluminum & Chemical Corporation, Oakland, Calif.

22 Filed: Oct. 5, 1970 211 App]. No.: 78,175

[52] U.S. Cl. ..423/121, 423/84, 423/85, 423/140 [51] Int. Cl. ..C01f7/06, C01f 7/34 [58] Field of Search ..23/l43; 423/121 [56] ReferencesCited UNITED STATES PATENTS 12/1900 Hall ..23/143 10/1931 Muller et a1...23/143 X FOREIGN PATENTS OR APPLICATIONS 3,195 7/1879 Great Britain..23/l43 OTHER PUBLICATIONS Edwards, The Aluminum Industry, Vol. 1,pages.

Primary Examiner-M. Weissman A1t0rney-Paul E. Calrow, Harold L. Jenkinsand Andrew E. Barlay 71 ABSTRACT 8 Claims, No Drawings BACKGROUND OF THEINVENTION This invention relates to a process for the recovery ofcaustic-soluble alumina values from aluminiferous ores. Moreparticularly, it concerns a digestion process for aluminiferous orescontaining titanium and also iron in the goethite phase with a calciumcompound to obtain caustic-soluble alumina in increased yield.

Alumina values can be recovered from aluminiferous ores in various ways.One widely utilized process is the well known Bayer process whichinvolves digestion of the aluminiferous ore in a caustic solution attemperatures between 200255 C. The digestion results in a causticaluminate solution and a caustic-insoluble residue commonly called redmud. In the usual Bayer process the caustic aluminate solution isseparated from the red mud and subjected to precipitation to enablerecovery of the alumina values. The red mud, which also containsoccluded or trapped causticsoluble alumina values, is generally washedto remove most of these values from the mud in order to improve theyield of the Bayer process.

In many instances the red mud will contain considerable quantities ofalumina which under the conditions of the Bayer process is unavailableto the caustic extraction process and constitutes unrecoverable values.Additionally, many aluminiferous ores when subjected to the wet causticextraction of the Bayer process yield a red mud of extremely fineparticles size, usually less than micron an often less than one micron.This red mud is hard to settle, and the washing of this type of mudbecomes time consuming and expensive. This is particularly true forthose aluminiferous ores which contain a portion of their iron contentin the goethite phase.

Goethite [(FeO)Ol-l] is commonly associated with diaspore, analumina-containing mineral, and it is believed that some of the Al atomsare within the lattice structure of goethite. The combination ofgoethitediaspore can be frequently found in Jamaican bauxites; however,there are many other bauxite sources, for example, Greece, Hungary,South America, which exhibit this type of combination of the iron andaluminum constituents.

It has also been found that unless the goethite is transformed to adifferent iron phase during the wet caustic digestion, for example, tohematite, the goethite lattice bound alumina will not be available forcaustic extraction.

The goethite content of the aluminiferous ores may vary between a fewpercent and up to about 90 percent by weight of the iron content of theore. Although the alumina values bound in the goethite lattice may be aslow as about 0.1 percent by weight of the total alumina of the ore, morefrequently, values in the range of about 2-3 percent by weight are foundand, in exceptional instances, values as high as 8-10 percent by weightcan be encountered. This lattice bound alumina, unless made available tothe caustic extraction process, will become lost in the red mud, causinga commerically significant reduction of yield even if the lattice boundalumina is only about 1.0 percent of the total alumina.

The following Table shows Jamaican bauxite samples containing iron inthe goethite phase and the percentage of lattice bound alumina.

TABLE I Lattice Goethite Content Bound Alumina Sample Weight Bauxite inWt. k of Total Alumina I 22.9 9.2 2 2L9 6.7 3 9.0 2.2

Conversion of the goethite phase to hematite can be accomplished, forexample, by raising the temperature of the digestion to about 260C, orabove, which will aid the effect of the caustic on the goethite lattice,resulting in increased alumina yields. Commerically, a suitable methodoperating at temperatures in excess of 260C and at elevated pressuresassociated with these temperatures is the so-called tube digestermethod. The digestion of the aluminiferous ores by this method isusually accomplished at temperatures in excess of 290C and at highpressure in a long heated tube to assure sufficient residence time andallow good contact between the caustic and the aluminiferous ores. Thus,this type of digestion process allows the conversion of the goethite toother iron phases and the lattice bound alumina can be made available tocaustic extraction. This process not only promotes the production ofalumina in increased yield due to the alumina recovered from thegoethite lattice but also results in a red mud which settlescomparatively faster than the red mud obtained from the regular Bayerprocess. Consequently, the mud can be washed substantially free ofcaustic soluble alumina values with less difficulty.

The high temperature digestion process (usually in excess of 280-290C)is capable of extracting alumina values from aluminiferous orescontaining a portion of their iron content in the goethite phase.However, it has been found that only a low degree of conversion ofgoethite to hematite occurs, even at these high temperatures andpressures if the aluminferous ore contains titanium, and thus only asmall increase in the yield of caustic soluble alumina can be obtained.Even if the temperature of the digestion is raised to, for example,350C, only a slight increase in the goethite to hematite conversion canbe observed in the presence of titanium. Consequently, the yield ofalumina values from aluminiferous ores containing both goethite andtitanium is considerably lower than yields obtained from ores devoid oftitanium. Thus, for example, when high temperatures, i.e., in excess of280C are utilized for the digestion of goethite and titanium containingaluminiferous ores without employing added lime quantities, only about20-30 percent of the lattice bound alumina can be recovered as causticsoluble alumina. With the process of the invention, up to about -90percent of the lattice bound alumina will be converted to causticsoluble alumina. Similarly, if the entire digestion is accomplished attemperatures below about 260C in the presence of a calcium compound, thegoethite to hematite conversion in the presence of titanium will beinsignificant, and conversion of only about 5-15 percent of the latticebound alumina to caustic soluble alumina will be observed. Thus, itbecomes evident that high temperatures associated with the addition of acalcium compound in sufficient quantity to titanium containingaluminiferous ores wherein a significant portion of the iron phase isgoethite, results in a substantial and commercially significant increasein caustic soluble alumina.

cess which allows recovery of the goethite lattice bound alumina fromtitanium-containing ores in order to improve the yields of the wetcaustic extraction and render the extraction process more economical.

The following Table presents some typical bauxite compositionsreproduced from Extractive Metallurgy of Aluminum, Volume 1, Alumina.Interscience Publishers, 1963.

TABLE II Analyses of Bausxites Derived from Various Types of RocksTotal..." i 100 i 100 l 100 i 100 99 98. 7 99. 7 98.6

Gibbsitic bauxite derived from nepheline syenite, Kassa, Guinea.Analyzed by Aluminuim Laboratories Ltd., Arvida, Canada. Gibbsitiebauxite derived from basalt, Fiji. Analyzed by Aluminium LaboratoriesLtd., Canada.

Gibbsitie bauxite derived from hornblende schist, Mackenzie, Britishguiana. Analyzed by Demerara Bauxite Co. Ltd., Mackenzie, British uiana.

d Gibbsitie bauxite derived from phyllite, Johore, Malaya. Analyzed bymunicipal analyst, Singapore.

9 Glbbsitic bauxite derived from mica schist, Darling Range, WesternAustralia. Analyzed by Aluminium Laboratories Ltd., Arvida, Canada.

Phosphatie gibbsitic bauxite derived from limestone, Fiji. Analyzed byAluminium Laboratories Ltd., Arvida, Canada.

B Boehmltie bauxite derived from limestone, Istria, Yugoslavia. Analyzedby Aluminium Laboratories Ltd., Arvida, Canada.

h Diasporic bauxite derived from limestone, Distomon District, Greece.Analyzed by Aluminium Laboratories Ltd., Arvida, Canada.

1 Alumina by difference.

It has now been discovered that alumina values can be recovered inincreased yields from goethite-titanium containing aluminiferous ores bydigesting the ore in admixture with an added calcium compound capable ofcombining with the titanium constituent of the ore generally as calciumtitanate, the digestion being conducted in a caustic medium attemperatures in excess of about 280C.

While the exact mechanism of the reactions taking place duringextraction of alumina in the presence of calcium compounds fromgoethite-titanium containing ores is not known, it is believed, withoutbeing bound to any particular theory, that in the presence of titanium,iron present in the ore in divalent form, will be bound to titaniumperhaps in the form of FeTiO When the titanium is converted to calciumtitanate, the divalent iron is believed to be available to catalyse theconversion of goethite to hematite, rendering the geothite lattice boundalumina available to the caustic extraction process, resulting inincreased yields. Other theoretical considerations present themselves,but in any event the use of calcium compounds in accordance with thenovel process of the present invention substan-- tially reduces thedeleterious effect of naturally present titanium on solubilization ofalumina.

BRIEF SUMMARY OF THE INVENTION A process for increasing the yield ofcaustic soluble alumina from aluminiferous ores containing titanium anda portion of the iron content in the goethite phase by caustic digestioncomprising the steps of admixing the aluminiferous ore with an amount ofa calcium compound at least sufficient to provide a calcium to titaniumatomic ratio of 0.5 to l, and with a caustic solution and treating theresulting slurry at temperatures in excess of about 280C.

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to aprocess for the recovery of alumina from aluminiferous ores. Moreparticularly, the present invention provides an improved process for therecovery of caustic soluble alumina in increased yield fromtitanium-containing aluminiferous ores.

For purposes of this invention, the expression aluminiferous ores refersto alumina-containing ores which, when subjected to a treatment with acaustic solution or liquor, yield alumina in the form of causticaluminate. Representative but not limiting examples of the ores suitablefor accomplishing the process of the invention include bauxite,diasporic clays and laterites.

The term caustic solution or liquor used hereinafter refers to asolution of sodium hydroxide or potassium hydroxide, sodium or potassiumaluminate containing liquors such as the spent liquor obtained from theBayer process or spent liquors concentrated by evaporation or byaddition of caustic, or by both. Aluminiferous ores generally containtitanium in amounts from about 1 to about 5 percent by weight(calculated as TiO and it is believed that during extraction asubstantial portion of the titanium is a titanium-iron compound.

In the process of the present invention the titaniumcontainingaluminiferous ore is generally admixed with a calculated amount ofcalcium compound. it is also possible to admix the components during orafter the preparation of an ore-caustic slurry. The calcium compoundssuitable for the conversion of the titanium in accordance with thepresent invention include calcium oxide, calcium hydroxide in slurryform, calcium carbonate and other basic calcium components, either indry form or as slurry. The ore can be predried to the desired moisturecontent, and if the ore is in large lumps, it is recommended to grindthe ore. The amount of calcium compound added to the ore should be atleast sufficient to allow conversion of at least a portion of thetitanium to calcium-titanate. In other words, the amount of calciumpresent in the admixture (taking into consideration the calcium contentof the ore, if any,) should be at least sufficient to establish an 0.5:1

calcium to titanium atom ratio, preferably an 0.8l:l

calcium to titanium atom ratio. Aluminiferous ores usually containquantities of other components which readily combine with calcium.compounds. Such components include, for example, P,O,etc., which can bepresent up to about 15-20 percent by weight of the ore. When the minimumamount of calcium compound necessary is to be determined, it isrecommended that these calcium consuming components be taken intoconsideration to assure substantially complete conversion of thetitanium to calcium titanate. The maximum amount to be added is dictatedby the economy of the process; however, it is generally advantageous toadmix the ore with a quantity of calcium, resulting in a 1:1 to 5:1 atomratio of calcium to titanium.

In accordance with the teachings of the present invention, thebauxite-calcium compound admixture is slurried to make it suitable forthe wet alkali extraction. The slurry is generally prepared by theaddition of a caustic solution, and while caustic solutions of the'typerecited above are all suitable for the slurry preparation,

for economy and expediency it is advantageous to utilize the spentliquor of the Bayer process for this purpose after its caustic contentis adjusted by the addition of NaOH.

In order to accomplish a reasonably rapid conversion of the titaniumcompounds, the slurry should advantageously contain 150-250 grams perliter caustic soda expressed as Na CO equivalent. Throughout thespecification, the caustic soda concentrations are expressed in gramsper liter and are reported as sodium carbonate, i.e., Na CO equivalent.The expression caustic soda includes the caustic soda combined with thealumina as sodium aluminate and the free caustic soda in the solution.The weight ratio of A1 to Na CO is generally expressed as A/C and forthe preparation of the slurry to be subjected to the novel firstdigestion step of the invention, A/C ratios of 0.35-0.70 arerecommended.

The slurry thus prepared can be preheated, if desired, or heated in thedigestion vessel. The novel digestion step is generally accomplished inpressure vessels capable of withstanding elevated temperatures andpressures employed by the process. Temperatures in excess of 280C,generally between 280-350C, are utilized for this step, preferablybetween 280-320C.

Under these conditions, the titanium will be converted to calciumtitanate and the lattice bound alumina will be rendered caustic solublewithin a short time period, generally less than 30 minutes. Naturally,the rate of conversion of the titanium depends on the temperatureutilized and the higher the temperature, the more rapid thetransformation.

Subsequent to the first digestion treatment, the slurry can be cooled tothe conventional Bayer process digestion temperatures, e.g., below260-255C. The cooling can be accomplished in several ways, such as forexample by admixture of the high temperature slurry either with water orwith spent Bayer liquor. Combination of flash-cooling and dilution with,for example, spent liquor can also be utilized.

The predigested slurry is then further digested by the conventionalBayer process, if desired, by the addition of more caustic, and aftercompletion of the second digestion step the slurry is utilized for therecovery of caustic soluble alumina values in a known manner.

It has been found that if first stage digestion temperatures betweenabout 280-320C are utilized, conversion of goethite to hematite can beachieved with corresponding conversion of the titanium values to CaTiOin less than 20 minutes, generally about lO-l 5 minutes. It has alsobeen found that if the second digestion stage is conducted attemperatures below about 255C, for example, between about 240C and 255C,time periods between -30 minutes suffice to dissolve the caustic solublealumina from the ore. Thus, if a two-stage digestion process inaccordance with the present process is utilized, total digestion timeperiods between about -45 minutes will provide the desired increase incaustic soluble alumina yields. Naturally, if the entire digestionprocess is carried out at temperatures in excess of about 280C, also inaccordance with the invention, total digestion times of about 20-30minutes will allow recovery of alumina in increased yields.

It has been found that the short term, high temperature first digestionstep not only allows recovery of alumina values in increased yield butalso the red mud resulting from the combined first and second digestionstep will possess better settling properties than the red mud of bauxitesubjected only to the conventional Bayer process. 7

It has been found that when titanium-containing bauxites are subjectedto the novel first digestion step in admixture with calcium compoundsatthe temperatures indicated above, the yield of recoverable causticsoluble alumina, i.e., total available alumina, increased generally by1-3 percent over the yield obtained from bauxites not treated inaccordance with the present process. Thus, it becomes evident that byutilizing the process of the present invention, the economy of the wetcaustic alumina extraction process can be considerably improved withoutthe necessity of carrying out the entire digestion process attemperatures in excess of 280C, which not only requires significantcapital expenditures but poses operating problems such as themaintenance of high pressure systems for extended periods, hightemperature pumping systems, increased steam and heat requirements, etc.

The entire digestion process can be carried out at temperatures inexcess of 280C, for example, in a tube digester with the addition of asufficient quantity of calcium compounds capable of converting thetitanium to calcium titanate. The novel step of the present inventionthus provides not only an advance over the known Bayer aluminaproduction, but also improves the high temperature digestion processeswhich, while operating at temperatures in excess of 280C, are incapableof recovering a significant portion of the lattice bound alumina in thepresence of titanium without utilizing the novel process.

The first digestion step of the novel process can be accomplished eitherin a batchwise manner, or in a semi-continuous or continuous manner. Ina continuous operation, for example, one digester can be utilized forthe high temperature predigest, from which the predigested slurry can becontinuously introduced into a digester operating at the conventionalBayer process temperature and pressure after the average residence timeof the predigested slurry satisfies the requirement set for theconversion of the titanium to calcium titanate and of goethite tohematite.

The following illustrative examples further show the operation of thenovel process without, however, being intended to limit the scope of theinvention.

EXAMPLE I A Jamaican bauxite originating from the South Coast of Jamaicawas dried at C for 2 hours and analyzed for constituents, with thefollowing results:

Total A1 0 48.05 by wt.

Fe O 19.68

SiO 1.60

TiO 2.63

Loss of Ignition (LOI) 26.44% (at l000C for 30 minutes), balance, metaloxides such as V, Ni, Mg, Cr, Cu and Zn.

The caustic soluble or total available (TAA) alumina amounted to 43.2percent by weight as determined by caustic digestion in absence of CaOat 246C. Lattice bound alumina was calculated to be 2.9 percent and thegoethite content of the iron of the ore was found to be 21.9 percent.

The dried bauxite was ground to a particle size wherein at least 90percent by weight passed through a screen having openings of 0.177 mm.105 grams of the ground bauxite was admixed with 2.06 grams of finelyground CaO and slurried with a caustic solution containing 172 g/lcaustic (calculated as g/l Na CO The slurry had an alumina to caustic(A/C) ratio of 0.4. The slurry was then introduced into a high pressureNickel 601 alloy autoclave and rapidly heated to 300C and kept at thistemperature for 30 minutes, then flashed to about 95C and diluted to itsoriginal volume for settling. The red mud was separated from the liquorand after washing was analyzed. It was found that all of the iron ingoethite phase was converted to hematite and from the residual aluminacontent of the mud, it was established that about 77 percent of thelattice bound alumina was extracted and made available for aluminaproduction. Analysis of the red mud also showed that the substantialquantity of titanium was converted to calcium titanate. The overallalumina extraction yield was increased by about percent.

EXAMPLE 11 A sample of the same bauxite used in Example I was subjectedto the same treatment as described in Example l, without the addition ofCaO to the ground bauxite. The red mud resulting from the digestion wasanalyzed, with the following results. X-ray diffraction studiesindicated that only a small fraction (less than about '20 percent byweight) of the goethite was converted to a. subjecting the ore to afirst stage digestion as a slurry in a caustic solution containing aboutto 250 grams per liter caustic soda, calculated as Na CO in admixturewith a calcium compound selected from the group consisting of CaO,Ca(Ol-l) and CaCO the amount of calcium compound added being in therange sufficient to establish a calcium to titanium atom ratio range ofabout 0.5 to l, the digestion being carried out at temperatures fromabout 280C to 350C and for a time sufficient to convert substantiallyall of the goethite to hematite and to form calcium titanate therebyreleasing lattice-bound alumina;

b. subjecting the slurry from the first stage digestion to a secondstage digestion at temperatures from about 200C to about 350C for a timesufficient to recover substantially all of the recoverable causticsoluble alumina from the ore; and

c. separating the caustic soluble alumina from caustic insolubleresidues and recovering the alumina values in an increased yield.

2..Process according to claim 1, wherein the second stage digestion iscarried out at temperatures below about 255C and in excess of 200C.

3. Process according to claim 1, wherein the first stage digestion isaccomplished at temperatures from about 280C to about 320C. I

4. Process according to claim 1, wherein the first and second digestionsteps are carried out at temperatures within the range of about280-350C.

5. Process according to claim 1, wherein the calcium to titanium atomratio is at least about 0.8 to l.

6. Process according to claim 1, wherein the first stage digestion isaccomplished in less than about 30 minutes.

7. Process according to claim 1, wherein the first and second stagedigestion steps are accomplished in less than about 45 minutes.

8. Process according to claim 4, wherein the first and second digestionsteps are accomplished in less than about 30 minutes.

2. Process according to claim 1, wherein the second stage digestion is carried out at temperatures below about 255*C and in excess of 200*C.
 3. Process according to claim 1, wherein the first stage digestion is accomplished at temperatures from about 280*C to about 320*C.
 4. Process according to claim 1, wherein the first and second digestion steps are carried out at temperatures within the range of about 280*-350*C.
 5. Process according to claim 1, wherein the calcium to titanium atom ratio is at least about 0.8 to
 1. 6. Process according to claim 1, wherein the first stage digestion is accomplished in less than about 30 minutes.
 7. Process according to claim 1, wherein the first and second stage digestion steps are accomplished in less than about 45 minutes.
 8. Process according to claim 4, wherein the first and second digestion steps are accomplished in less than about 30 minutes. 